As an NOx catalyst arranged in an exhaust system of a diesel engine or the like, a selective catalytic reduction (SCR) catalyst is known which selectively reduces and purifies NOx contained in exhaust gas with ammonia (NH3). Ammonia is used as a reducing agent, and is generated by hydrolysis from an aqueous urea solution (urea water).
If an excessive amount of aqueous urea solution is injected into the SCR catalyst, and an amount of NH3 supplied to the SCR catalyst exceeds NH3 adsorption capacity of the SCR catalyst, then unwanted slip of excess NH3 occurs, and the excess NH3 is emitted to the atmosphere. This is not a desirable situation. Accordingly, there is a known technique of estimating the adsorbed amount of NH3 in the SCR catalyst on the basis of a detection value of an NH3 sensor provided at an outlet of the SCR catalyst, and correcting as necessary an amount of aqueous urea solution to be injected into the SCR catalyst in accordance with the estimated adsorbed amount of NH3 (see, for example, Patent Literature Document 1).